The present invention relates to free piston engines, and more particularly to free piston engines having an opposed piston opposed cylinder configuration.
Conventionally, internal combustion engines have operated with the motion of the pistons mechanically fixed. For example, a conventional internal combustion engine for a motor vehicle includes a crankshaft and connecting rod assemblies that mechanically determine the motion of each piston within its respective cylinder. This type of engine is desirable because the position of each piston is know for any given point in the engine cycle, which simplifies timing and operation of the engine. While these conventional types of engines have seen great improvements in efficiency in recent years, due to the nature of the engines, that efficiency is still limited. In particular, the power density is limited because the mechanically fixed motion of the pistons fixes the compression ratio. Moreover, all of the moving parts that direct the movement of the pistons (and camshafts and engine valves as well) create a great deal of friction, which takes energy from the engine itself to overcome. The resulting lower power density means that the engine will be larger and heavier than is desired. Also, the flexibility in the engine design and packaging is limited because of all of the mechanical connections that must be made.
Consequently, is desirable, for environmental and other reasons, to have an engine with a higher power density than these conventional engines. The advantages of lighter relative weight, smaller package size, and improved fuel efficiency can be a great advantage in both vehicle and stationary power production applications.
Another type of internal combustion engine is a free piston engine. This is an engine where the movement of the pistons in the cylinders is not mechanically fixed. The movement is controlled by the balance of forces acting on each piston at any given time. Since the motion is not fixed, the engines can have variable compression ratios, which allow for more flexibility in designing the engine's operating parameters. Also, since there are no conventional crankshafts and rods attached to the crankshaft, causing piston side forces, there is generally less friction produced during engine operation.
An opposed piston, opposed cylinder (OPOC) configuration of a free piston engine is desirable due to its generally inherently balanced operation, with a compact layout as well. A particularly advantageous way to operate such an engine is with a two stroke combustion cycle, which requires a means for providing sufficient air to the cylinder. Moreover, it may be desirable to operate in a two cycle mode with homogeneous charge, combustion ignition (HCCI) combustion, which takes best advantage of the ability to operate with a variable compression ratio, further increasing the power density and reducing emissions and fuel consumption of the engine. HCCI operation at lean air fuel ratios (44:1 or leaner) reduces NOx emissions to a low value—however this requires an even greater ability to maximize the air intake for each engine stroke in order to produce maximum output power. This high volume of air intake ability, then, is desirable, while still being able to maintain a relatively compact and lightweight engine.
In a conventional two stroke engine, the back side of the piston—facing into the crankcase—is sometimes employed as a scavenge pump. But with an OPOC free piston engine, there is no crankcase, and further, the cross sectional area of the back side of the piston may not provide sufficient pumping capacity for obtaining the desired air charge in the combustion cylinder.